The present invention relates to a push-pull power amplifier with quiescent current regulator, in particular for low-frequency power amplifiers of high quality.
To ensure a linear mode of operation of the output transistors of a power amplifier from very weak output signals up to maximum output an operating point must be determined. The main problem with a push-pull output stage is keeping the desired quiescent current constant over a large temperature range.
In conventional power amplifiers in the operating point regulator a component is used which is thermally connected to output transistors. The thermal variations of an electrical value of the component are utilised to compensate changes of the quiescent current of the output stage.
Another method is the direct detection of the quiescent current. Its measured value is taken from a resistor in series with output transistors. The resistor is bridged by a diode to prevent the voltage drop in full-output operation from being too high. As measured value a mean value of the voltage at the resistor is assumed, which remains approximately constant in full-output operation.
When employing field-effect transistors for power amplification use is made of their property that for a given gate-source voltage U.sub.GSO their drain current is independent of the temperature. This voltage is kept constant.
In pulse-width modulation (PWM) power amplifiers the low-frequency input signal is converted to a digital form, processed and after reconvertion generated as an amplified signal at the output.
In the first method, the quiescent current, because of the spread of the parameters of the semiconductors used in the amplifier, must be set by hand. This is very time-consuming. Another disadvantage is the low long-time stability of the variable resistor compared with the fixed resistors. The sluggishness of the reaction on temperature increases in the transistor crystals of the output transistors is caused by poor thermal coupling with the sensor element. The additional stabilising steps in the form of resistors in series with the connected load reduce the output power obtainable.
The second method, although it avoids the aforementioned disadvantages, in the final result hardly improves the stability of the quiescent current. The influences of the ambient temperature, operating voltage, spread between units of the semiconductors used, amplitude and curve form of the output signal are not negligible.
In the third method the quiescent current is determined by the semiconductor manufacturers themselves and is optimal only from the point of view of temperature independence. Moreover, it is practically impossible for the U.sub.GSO voltage of the two complementary output transistors to be the same. The voltage is different even with different units of the same types of transistors.
PWM amplifiers require an enormous technological expenditure and are very costly. An LC filter network used for reconversion of the digital PWM signal considerably increases the output impedance of the amplifier. Also very critical are interference radiations extending into radio and television receiving ranges as well as sound-distorting intermodulation between the switching frequency and the signal frequencies.
The objective of the present invention is to keep fixed with greater accuracy compared with known circuits the quiescent current of the output transistors in view of the unit spread of the semiconductors, temperature and operating voltages, etc., used in the amplifier.
This problem is solved according to the invention in a push-pull power amplifier with quiescent current regulator, in particular for low-frequency power amplifiers of high quality in that the quiescent current or currents of the output transistors is or are sampled at a predetermined output voltage and/or in a predetermined output voltage range of the amplifier, their values coded in one or more comparators and in one or more storage elements the corresponding values with which the desired values of the quiescent current or currents can be corrected or held are stored.
This gives several very important advantages:
1. No thermal feedback to the output transistors is necessary. PA0 2. No adjustments are necessary. PA0 3. Low number of components. PA0 4. Very high accuracy of the stabilising (with respect to operating voltage and temperature changes, etc.). PA0 5. No quiescent current flows when the amplifier is switched on and the output transistors are not driven at full power. In conventional amplifiers a short-circuit current often flows here. PA0 6. Simple integration possibility of the control stage. PA0 7. Simple realisation of short-circuit protection. PA0 8. Possibilities of digital processing of the quiescent current, e.g. digital storing of a corresponding measured value. Variation of the quiescent current depending on the magnitude of the output signal, warning signalisation before overheating of the output transistors, etc.